The invention relates to a powder compacting apparatus for forming a helical gear wheel by compressing powder and to a method of forming a helical gear wheel using such powder compacting apparatus.
In the case where a helical gear wheel is formed using a powder compacting apparatus having an upper punch, a lower punch, a die, and a core rod, the upper punch and the lower punch that are involved in the forming of helical gear, as well as the die or the core rod that moves relative to the upper punch and the lower punch in axial directions while meshed with the upper punch and the lower punch through the helical gears must be relatively rotated in accordance with a lead of the helical gear during the powder compression process in which a powder material is compressed in the die and during the withdrawal process in which a green compact obtained after the completion of the powder compression process is ejected out of the die.
As means for relatively rotating the upper and lower punches and the core rod, the following methods are known. (1) A die or a core rod having a helical gear contour for forming a helical gear is fixed and an upper punch and a lower punch are rotated. (2) A die or a core rod having helical gear for forming and an upper punch are rotated and a lower punch is fixed.
FIG. 1 outlines a powder compacting apparatus adopting method (1), and FIGS. 2 and 3 outline powder compacting apparatuses adopting method (2). FIGS. 1 and 2 show powder compacting apparatuses for forming internal helical gear wheels, in each of which helical gear contour G.sub.1, is formed on an outer circumference of a core rod 9, and helical gear contours G.sub.2, G.sub.3 meshable with the helical gear contour G.sub.1 are formed on inner circumferential portions of an upper punch 1 and a lower punch 7, respectively.
Further, FIG. 3 shows a powder compacting apparatus for forming an external helical gear wheel, in which helical gear contour G.sub.1, is formed on an inner circumferential portion of a die 5, and helical contours G.sub.2, G.sub.3 meshable with the helical gear contour G.sub.1 are formed on outer circumferential portions of an upper punch 1 and a lower punch 7, respectively.
It may be noted that the core rod 9 is fixed to a yoke plate 10 and the lower punch 7 on a base plate 8 is rotatably supported through a bearing 14 in FIG. 1. It may be further noted that the lower punch 7 is fixed onto the base plate 8 and the core rod 9 on the yoke plate 10 is rotatably supported through the bearing 14 in FIG. 2. Reference numeral 15 in FIG. 2 denotes guide pins engaged with grooves which have the same lead as that of the helical tooth G.sub.1 portion. The core rod 9 rotates so as to correspond to the lead of the helical gear contour G.sub.1 when the core rod 9 ascends while guided by the guide pin 15.
The lower punch 7 is fixed and the die 5 is rotatably supported in FIG. 3. The die 5 is constructed so that the lead phase thereof is adjusted with reference to the lower punch by engaging a guide groove 16 (having the same lead as the helical gear contour G.sub.1 disposed on the lower punch 7 with the guide pin 15. When the die 5 ascends, the die 5 is rotated while guided by the guide groove 16.
The upper punch 1 is rotatably supported in both apparatuses so that the helical gear contour G.sub.2 can be meshed with the helical gear contour G.sub.1 correctly. A guide groove 18 whose lead is identical to the lead of helical gear contour G.sub.1 is also disposed on the upper punch 1. The upper punch 1 starts rotating from a predetermined position while descending and while being guided by the guide pin 17 that is engaged with the guide groove 18. That is, the upper punch 1 starts rotating from a position at which the guide plate 4 that is descending up to some position together with the upper plate 2 has movement regulated by the guide stopper 13. Then, the upper punch 1 meshes with the helical contour gear G.sub.1 and is pushed into the die 5.
As a result of the upper punch 1 having been pushed into the die 5, a powder material M in a cavity formed in the upper punch 1, the die 5, the lower punch 7, and the core rod 9 is compressed into a green compacts A. The green compact A is ejected out of the die 5 by the forced lowering from the compression-completed points of the die 5 and the core rod 9 (the lowering of the die 5 and the core rod 9 prior to the forced lowering takes place spontaneously).
It may be noted that some of the apparatuses adopting method (2) take care of elastic distortions during compression of the upper and lower punches (Unexamined Japanese Patent Publication No. Hei. 7-150204).
The apparatus adopting method (1) (e.g., the apparatus shown in FIG. 1) ejects the green compact A out of the die 5 while rotating the green compacts A. Therefore, the green compacts A is chipped and cracked.
To overcome this problem, the apparatus adopting method (2) that can eject the green compacts A out of the die without rotating the green compact A is often used. However, this apparatus has the following shortcoming. If the lower punch 7 is long or slender, the lower punch 7 is elastically deformed and largely flexed due to a load applied thereto during compression, which in turn causes a lead-phase shift between the helical gear contours G.sub.1 and G.sub.3. As a result, an unintentional, excessive force is applied to the meshed portion between both helical gear contours G.sub.1 and G.sub.3 and the engaged portion between the guide pins 15, 17. Hence, the die assembly may, in some cases, be broken.
To overcome this shortcoming, Unexamined Japanese Patent Publication No. Hei 7-150204 has disclosed an apparatus in which elastic distortions of the upper and lower punches are sensed by sensors so that rotation of the upper punch is corrected by adjusting the height of the guide plate during compression. This apparatus can prevent errors in meshing the helical gears of the upper and lower punches with the helical gear of the die by correcting a lead-phase shift between the helical gears of the upper and lower punches while correcting the rotation of the upper punch. Therefore, the problem of die assembly breakage can be overcome. However, a distortion sensor, a guide plate height adjusting mechanism, and a device for controlling a drive source of the height adjusting mechanism based on an output of the sensor must be additionally provided, which disadvantageously complicates the structure of the powder compacting apparatus and increases the price thereof.
Further, the lower punch flexed due to compression returns when the pressure is released after the compression. Therefore, the green compact is raised in the die and rotates due to restitution of the lower punch from flexure. As a result, the green compact is not completely free from chipping and cracking. This shortcoming is addressed also in the apparatuses shown in FIGS. 2 and 3.